Saturday, February 24, 2007

Reporting findings that help shape our understanding of how tool use has evolved among primates, researchers have discovered evidence that chimpanzees, at least under some conditions, are capable of habitually fashioning and using tools to hunt mammalian prey. The work [1], reported by Jill Pruetz of Iowa State University and Paco Bertolani of the University of Cambridge, will appear online in the journal Current Biology on February 22nd 2007.

Chimpanzees are well known for their ingenuity in using tools for some tasks, such as obtaining invertebrate insects from logs or pounding open hard nuts, but there had been only fleeting evidence of chimpanzees brandishing tools for bona fide hunting.

In the new work, researchers observed tool use in hunting by the Fongoli community of savanna-dwelling chimpanzees (Pan troglodytes verus) in southeastern Senegal. Chimpanzees were observed making spear-like tools in a step-wise fashion, and subsequently using them with jabbing motions in an apparent effort to obtain lesser bushbabies (Galago senegalensis) from cavities in hollow branches or tree trunks. Bushbabies are nocturnal prosimians that retire to such hidden cavities during the day.

Although there was only one successful attempt in 22 recorded instances of the chimpanzees using the spear-like tools to find and obtain prey, the researchers observed that tool-crafting and associated hunting behavior was systematic and consistent, suggesting that it was habitual. The hunting behavior included forceful jabbing motions into branch or trunk hollows, and chimpanzees were seen to subsequently open the hollows by breaking wood off from a distance, suggesting that the jabbing actions were intended to immobilize bushbabies, rather than rouse them from their cavities (bushbabies move quickly and might otherwise easily evade chimpanzees once roused).

Two notable aspects of the behavior observed in the Fongoli group were that on the one hand, it is rare for chimpanzees to consume prosimian prey - in other study sites, red colobus monkeys, hunted mainly by males, are the chimps' most common prey - and on the other hand, the tool use appeared to be primarily restricted to females and immature individuals. These two behavior characteristics could both be related to the fact that the Fongoli community inhabits a mosaic savannah that is relatively dry, and where red colobus monkeys are absent. This habitat may promote efforts - such as the observed tool use - to obtain meat through other means.

The authors point out that the females and immature chimpanzees using the spear-like tools appear to be exploiting a niche relatively ignored by males, an observation that supports a previous hypothesis that female hominids played a role in the evolution of the earliest tool technology and suggests that this technology may have included tools for hunting. [Primatology, Anthropology]

Although tool use is known to occur in species ranging from naked mole rats [1] to owls [2], chimpanzees are the most accomplished tool users [3, 4, 5]. The modification and use of tools during hunting, however, is still considered to be a uniquely human trait among primates. Here, we report the first account of habitual tool use during vertebrate hunting by nonhumans. At the Fongoli site in Senegal, we observed ten different chimpanzees use tools to hunt prosimian prey in 22 bouts. This includes immature chimpanzees and females, members of age-sex classes not normally characterized by extensive hunting behavior. Chimpanzees made 26 different tools, and we were able to recover and analyze 12 of these. Tool construction entailed up to five steps, including trimming the tool tip to a point. Tools were used in the manner of a spear, rather than a probe or rousing tool. This new information on chimpanzee tool use has important implications for the evolution of tool use and construction for hunting in the earliest hominids, especially given our observations that females and immature chimpanzees exhibited this behavior more frequently than adult males.

In the early 1960s, when the british primatologist Jane Goodall first observed wild chimpanzees hunting and eating meat in Gombe National Park, Tanzania, it was widely believed that these animals were strict vegetarians. Skeptics suggested that the diet of the Gombe chimpanzees was aberrant. Others suggested that the quantity of meat the chimpanzees ate was trivial. After more than 30 years of research, however, it is now clear that meat is a natural part of the chimpanzees' diet. Indeed, hunting has been observed at most of the other sites where chimpanzees are studied across central Africa. And, it turns out, a chimpanzee community may eat several hundred kilograms of meat in a single year.

To many anthropologists this is a surprising development. Of all the higher primates, only human beings and chimpanzees hunt and eat meat on a regular basis. The similarities pose an intriguing prospect: Might the close evolutionary relationship between chimpanzees and human beings provide some clues to the evolution of our own behavior? We do know that the earliest bipedal hominids, the australopithecines, evolved in Africa about 5 million years ago and that they shared a common ancestor with modern chimpanzees shortly before that time. Unfortunately, the evidence for the occurrence of meat-eating among the early australopithecines is spotty at best. Primitive stone tools that were made 2.5 million years ago suggest that early hominids had the means to carve the flesh from large carcasses, but we know very little about their diets before that time. Were they hunters or perhaps, as many anthropologists now argue, scavengers? The behavior of chimpanzees may provide a window through which we can see much that has been lost in the fossil record.

There are also some interesting subtleties to the chimpanzees' hunting behavior that need to be addressed. Although chimpanzees can and do hunt alone, they often form large hunting parties consisting of more than 10 adult males, plus females and juveniles. Chimpanzees also go on "hunting binges" in which they kill a large number of monkeys and other animals over a period of several days or weeks. Such binges have always been a little mysterious. What could incite a chimpanzee to suddenly forgo plant foraging and turn to hunting? Are there social or ecological factors associated with the impetus to hunt? What ecological effects does the chimpanzees' predatory behavior have on their prey?

In the past five years I have been mindful of such questions as I observed the chimpanzees and their primary prey at Gombe, the red colobus monkey. Although we are only beginning to understand some of the causes and consequences of the chimpanzees' actions, what we have discovered is more complicated and more interesting than anyone suspected. For chimpanzees, meat is not only another way to get nutrients like fat and protein, but a means to make political bonds and gain access to sexually receptive females.

David Attenborough video clip* showing how hunting chimpanzees co-ordinate their behavior in order to trap and kill (then eat) a colobus monkey:

*From The Trials of Life: A Natural History of Behaviour, a BBC nature documentary first broadcast in 1990

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The video footage below hasn't any commentary but is accompanied by the following text by kambizkamrani: "Adolescent female Tumbo isolates a potential spear and modifies it. She begins to jab it into a tree to spear her prey. She then climbs the tree and begins jumping on the large limb, which eventually breaks off, allowing her to reach in and retrieve the prey, a bushbaby (Galago senegalensis)." - watching the National Geographic video "Video: Chimps Make and Use 'Spears' to Hunt" first may help.

Nice post which there was only one successful attempt in 22 recorded instances of the chimpanzees using the spear-like tools to find and obtain prey, the researchers observed that tool-crafting and associated hunting behavior was systematic and consistent, suggesting that it was habitual. The hunting behavior included forceful jabbing motions into branch or trunk hollows, and chimpanzees were seen to subsequently open the hollows by breaking wood off from a distance, suggesting that the jabbing actions were intended to immobilize bushbabies, rather than rouse them from their cavities. Thanks a lot for posting this article.